Nickel-metal hydride storage (rechargeable) batteries are widely used as power sources of microelectronic equipment such as digital cameras and laptops because of their high energy density, and used as alternative power sources to primary batteries such as alkaline manganese batteries because the nickel-metal hydride storage batteries are equal in the operating voltage to the primary batteries and are compatible with the primary batteries. Therefore, demands for the nickel-metal hydride storage battery are steadily expanding.
Such a nickel-metal hydride storage battery uses a hydrogen storage alloy as a negative active material. An AB5-type rare earth-Ni-based alloy having a CaCu5-type crystal structure has been put to practical use as a hydrogen storage alloy. MmNi5 is known as a typical example thereof.
However, the battery using the MmNi5-based alloy has a problem that the preservation property is not sufficient. The preservation property can be improved by using an electrolyte solution containing a large amount of NaOH. However, since the electrolyte solution containing a large amount of NaOH tends to corrode the hydrogen storage alloy compared with a commonly used electrolyte solution containing KOH as a main ingredient, there is a problem that the battery life is shortened (JP-A-9-139230).